Hydraulic cements are cements that set and develop compressive strength due to hydration. Such cements can therefore be set under water. As such, hydraulic cements are often used for cementing pipes or casings within a wellbore.
In a typical cementing operation, casing is lowered into the wellbore. A cementitious slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to set and harden to hold the casing in place.
Successful cementing of the well pipe and casing during oil and gas well completion requires the cement slurry to have several important properties. For instance, the cement slurry must be pumpable and capable of being set within a practical time. In addition, settling of cement particles within the slurry must be minimal. Further, the slurry should be capable of exhibiting fluid loss control.
Under normal conditions, cementitious slurries having a density of approximately 15.0 ppg are desired and the cement in such slurries is typically Portland cement. Such slurries quickly develop compressive strength upon introduction to a subterranean formation, typically within 48 hours from introduction. As time progresses, the cement develops greater strength while hydration continues.
In some locations, the subterranean zones or formations into or through which wells are drilled are weak and, as such, are characterized by high permeability, low compressive strength and low tensile strength. The resistance of such subterranean zones or formations to shear is therefore low and such zones or formations typically have very low fracture gradients. When a well fluid, such as a hydraulic cementitious slurry, is introduced into a wellbore penetrating such a subterranean zone or formation, the hydrostatic pressure exerted on the walls of the wellbore may exceed the fracture gradient of the zone or formation. Fractures may form in the zone or formation and the cementitious slurry may be lost in such fractures.
When formations are either unable to support high hydrostatic pressures or are highly permeable, it is desirable to use low density cementitious slurries. A lightweight cement exerts a lower hydrostatic pressure on the formation compared to conventional cements. Cementitious slurries containing lightweight cements are also used to reduce the numbers of stages required for cementing deep wells. Such slurries often therefore minimize operating costs and hazards associated with multistage cementing.
Certain specifications must be met in order for lightweight cement slurries to be acceptable replacements for conventional cementitious slurries. In addition to exhibiting suitable compressive strength and tensile strength, lightweight cement slurries must be easy to mix (no settling) and be easy to blend (no solid segregation).
Several methods are currently used to produce lightweight cement slurries. For instance, excess water may be added to the cementitious slurry for the purposes of lowering the density of the slurry. In other instances, the slurry may be foamed. In still other instances, density modifying extenders, such as hollow pozzolanic, glass spheres or synthetic spheres, may be added to the slurry.
Although each of these methods has produced lightweight cementitious slurries with some degree of success, none of these methods fulfill all the requirements stated above. For instance, while water extended cements render slurries having lower densities, they also produce lower quality cement. Foamed cementitious slurries present challenges in design and field execution, require special equipment (for example, high pressure cryogenic pumps and liquid nitrogen tanks) and highly skilled personnel. Further, when nitrogen is used, the base cement slurry can only be expanded by 25-30% while retaining the mechanical properties of the original slurry. Hollow pozzolanic spheres are generally unsuitable for cement slurries with densities lower than 10.0 ppg and typically will not resist hydrostatic pressures higher than 3,000 psi. Further, since such spheres are a by-product of coal burning operations, they exhibit significant variations in densities and a wide particle size distribution, which increases the difficulty of job design. Borosilicate glass spheres exhibit a much tighter particle size distribution but their extremely low density challenges job design and field execution. Further, the alternatives for lowering the density of the slurry presently offered all employ non-reactive lightweight additives. These additives do not contribute to, and may hinder, the development of strength and other desirable properties in the cement.
Alternatives have been sought for lightweight materials for slurries which may be used in the cementing of pipes and casings within a wellbore and which further improve the physical properties of the set cement and facilitates easy handling of the slurry.